In MIMO wireless communication systems, multiple spatial streams are simultaneously transmitted via multiple antennas of a first wireless device to a second wireless device. When applying beamforming weights to the signals that form the multiple spatial streams to be transmitted, knowledge of the channel information with respect to all of the antennas of the second wireless device is needed for accurate computation of the beamforming weights.
One technique is to implicitly estimate beamforming weights from received signals. Another technique is to use a dedicated and separate training transmission sequence. Implicitly determining beamforming weights is advantageous because it does not increase overhead and latency, and has minimal negative impact on channel coherency.
In order to implicitly determine downlink beamforming weights, it is necessary to receive as many uplink spatial streams as will be transmitted in the downlink to that destination device. There are various occasions in which a destination device may not use its' full uplink spatial capability, e.g., when using cyclic shift diversity, making it more difficult to extract the full dimensional spatial information needed.